Heretofore, nearly all the catalysts developed for the production of PTMEA from tetrahydrofuran (THF) were based on catalytic reaction over proton donor materials or in the presence of materials known to impart the Bronsted acidity to the reaction mixture. Among these catalysts, homogeneous catalysts that are present in the liquid phase have rarely been commercially exploited due to its prohibitive cost of the catalytic material as well as the inherent difficulty associated with the separation and recovery of the liquid-phase catalyst from the liquid product mixture. Therefore, the most promising catalysts in the commercial large-scale production of PTMEA have been of the heterogeneous type and commonly based on the natural halloysite mineral which is basically an aluminosilicate mineral with its SiO2-to-Al2O3 ratio being close to 2.0. The inherent Bronsted acidity of the aluminosilicate mineral becomes the highest when the SiO2-to-Al2O3 ratio is at or close to 2.0, which is theoretically attributable to the electron distribution and electronic stabilization of the mineral's crystalline lattice. Even though zeolite and other zeolite-like minerals are also based on aluminosilicates with silica and alumina in varying ratios and these types of minerals can also be synthesized, they have not been effectively used as the catalysts for the production of PTMEA due to multiple reasons that include: the high cost; the lack of other trace mineral matters that also contribute to the catalyzing Bronsted acidity; the unfavorable morphological structure involving pores and internal surface area, and more. Some of the prior efforts, including the Korea PTG Company's, involved doping or impregnation of synergistically positive mineral ingredients (such as hematite, other transition metal compounds, sodium salts, potassium salts, magnesium salts, calcium salts, etc.) onto the conventional halloysites in order to increase the Bronsted acidity of the resultant catalyst. However, their overall effects on the process efficiency of the polymerization of poly(tetramethylene ether) diacetate [PTMEA], a precursor to PTMEG, have been largely inconclusive, or minimally enhancing in certain areas but detrimental in other aspects of the process and product quality. Thus, the catalysts of the present invention are generally free of any zeolites. That is, if utilized, the amount of zeolite is generally less than about 10% by weight, desirably less than about 5% by weight, preferably less than about 2% by weight, and preferably nil, that is no zeolite is utilized based upon the total weight of the halloysite catalysts.
Poly(tetramethylene ether) glycol [PTMEG] is also known as polytetrahydrofuran (polyTHF) or poly(tetramethylene oxide) since a starting material of tetrahydrofuran (THF) is typically polymerized over an acid catalyst. Prior art before 1997 for polymerization of tetrahydrofuran (THF) to poly (tetramethylene ether) glycol (PTMEG) is described in U.S. Pat. No. 6,207,793 (by Sung-II Kim; assigned to Korea PTG Co.), hereby fully incorporated by reference.
U.S. Pat. No. 6,207,793 teaches the method and steps of polymerization over a halloysite catalyst and also teaches how to prepare the halloysite catalyst using certain types of naturally occurring mineral of halloysite, more specifically Korean halloysite. The role of the halloysite catalyst in this polymerization process is that it serves as a long-term and abundant source and donor of Bronsted acidity, i.e., supplying necessary proton (H+) to catalyze the desired reaction of polymerization of THF to polyTHF. To expedite this specific polymerization reaction, a small amount of alkyl anhydride such as acetic anhydride [(CH3CO)2O] can also be added to the reaction mixture. The amount of acetic anhydride that is used in the reaction mixture ranges between 0 to 20% of the starting amount of THF, more preferably 2-10%. The role of acetic anhydride is similar to that of an initiator in the case of free radical addition polymerization. If acetic aldehyde is added in the reaction mixture, the polymeric product from the polymerization reaction step would be poly(tetramethylene ether) diacetate [PTMEA], which is further converted into poly(tetramethylene ether) glycol [PTMEG] upon reaction with methanol. This invention is pertinent to the first step of THF polymerization and its catalyst.
Natural halloysites have a very large population of extra-small pores whose pore opening dimension is smaller than 20 Å. These tiny pores contribute to a large amount of internal surface area that could be useful when a gas phase reaction is to be promoted or catalyzed. However, such extra-small pores are dimensionally too small and largely inaccessible or impenetrable for large molecules that are typical for liquid phase chemical reactions involving growing polymers as in the case with the polymerization of PTMEA a precursor to PTMEG. In such a case, the dimension of growing molecules such as oligomers and polymers can easily exceed that of the pore opening, thereby creating an undesirable (in fact, disastrous) situation where larger polymer molecules get permanently entrapped inside the pores. Some of the molecules would be still growing and/or trying to grow further inside this tight space. The consequences of such improper growth and inhibited/prohibited diffusion of product molecules render nontrivial causes for a significant loss of and a diminishing effect on the catalyst activity, a waste in raw material, a shorter catalyst life, uneven polydispersity of product polymer, mechanical weakening and eventual structural breakdown of the catalyst, potential formation of high molecular weight gel, and more. Computation of molecular dimensions of oligomers is attached as Example 1. It explains why the existence of very small micropores in the catalyst is detrimental for the current process. This is somewhat contrary to the general thoughts about catalysts. It also signifies the effectiveness of the supercritical CO2 pretreatment, which is a main body of this invention.
The large population of extra-small inaccessible pores in the conventional halloysite catalyst also affects the efficiency of the acid wash ion exchange step by significantly slowing down the ion exchange process as well as resulting in grossly insufficient and inefficient surface coverage of Bronsted acid sites.
While halloysites, particularly Korean halloysites (only from a specific region of South Korea), after the acid wash ion exchange treatment, have been successfully exploited in commercial production of PTMEA/PTMEG at the Korea PTG production facilities (Ulsan, Korea), using the process technology based on its own patent of U.S. Pat. No. 6,207,793, the following areas have been identified as the areas requiring significant improvements and/or critical technological solutions:                1. Development of more efficient catalysts for faster polymerization reaction;        2. Smaller reactor volume required for lower capital and operational cost that leads to enhanced profitability;        3. Achieving higher product quality and better process controllability.        4. Raw material flexibility and resource diversification allowing for the use of worldwide halloysite minerals beyond the high-cost Korean halloysite that is currently being used;        5. Preparation of catalyst pellets possessing superior mechanical strength and attrition resistance in harsh reaction environments;        6. Allowing long-term reusability of the catalyst, and,        7. Achieving enhancement of overall process efficiency and profitability.        